The invention concerns an apparatus for reactively coating a substrate with an electrically insulative material, silicon dioxide (SiO.sub.2) for example. It comprises a power source for alternating current which is connected to an electrode disposed in an evacuable coating chamber. This electrode is electrically connected to a target which is sputtered. The sputtered particles are deposited on the substrate while a process gas and a reactive gas are supplied to the coating chamber.
In known processes for coating substrates by means of cathode sputtering where materials with a high affinity for the reactive gas are used, the problem arises that not only the substrate but also parts of the apparatus like the inner wall of the process chamber or parts of diaphragms are coated with materials of no or poor electrical conductivity. This requires a frequent change of process parameters, particularly electrical arcings, during one single coating procedure or even a frequent interruption of the process and even a frequent cleaning or replacement of parts of the apparatus.